Process and apparatus for cracking oil



Feb. 28, 1928.

E. c. HERTHEL PROCESS AND APPARATUS FOR CRACKING vIL original-Filed March 2s, 1925 y /7 lNvEN IOR El@ ene Her/d Patented Feb. 2s, 1928."

STATES 'OFFICE-1' cracking heavier hydrocarbon oils, such EUGENE c. HERTHEL, OF oHIc'AGO, ILLINOIS, AssIGNon 'ro sINcLAIR BEFINING GOM- v IANY,'OF NEW YORK, N. Y., f

A vClOIxfPOlthArElION F MAINE.

y PROCESS AND APPARATUS FOR CRACKING Application leanarclizs, 14925, seria'1 No. 1"z,5s7. nenwea .April 26,1927.

This invention relates to improvements in as gas oil, for the production-of lighter .hydrol "carbon oils, such as gasoline and pressure distillate, therefrom. The d. invention in- Y vcludes improvements apparatus; a

inboth method and In cracking heavier-hydrocarbon oils in externally heated pressure stills, theheat for the cracking operation is supplied' by heating gases, h ot products of `combustion from a furnace in which fuelis hurned,'which'are a plied directly to the heating surfaces of t e pressure still, directly to the walls-of the still in vthe case of a shell still and directly to the heating tubes inthe case-of a tube still. As thecrac'king operation progresses,

the formatllin the Oil charge of the still of carbon tar constituents. takes p lace, and, in stills in which the charge is reclrcu- "latedv over the heating surfaces, the extent to which the cracking operation can be carried in a single run is limited by theaccumulaj A tion of such constituents which'tend to settle or separate out on the heating surfaces 'and'to form a v layer ofv so-called carbon thereon Which'interferes with the 'heat trans- ,"fer, and, unless' periodicallyl removed .-by

plied to maintain the charge in a` suiiicie'ntly cleaning, causes overheating -of the walls Lor tubes ofthe still. To' avoid or minimize the danger of such carbonl separation onthel heating surfaces, the 'cracking -Op'erationis corn- `monly` stopped before the accumulation of heavy tar constitutents becomes too great, or the tar is withdrawnv and fresh feed supfluid condition to permit continued operation for alonger time. Thedanger of such carbon -separation and resulting failure of the vreatest where the heating gases fresh from the furnace and-at highest temperature first 'come in' contact with the heating surfaces' ofl the still and Where the greatest amount of heating takes place.

According to the present invention, thev heating of the` pressure still lis modified and l and eilicienc l mg furnace.

regulated to protect the heating surfacesv of the pressure still itself, to improve the cracking operation, and to increase the capacity y of thefpressure still and heat- According tothe present invention, the

j heating gases from the furnace are circulated fir-Stover an auxiliary heating' coil and are then admixed with hot gases at a lower temperature before being circulated over the i heating surfaces of the pressure still proper. 6

Oil is circulated through the auxiliary coil and the auxiliary coil is employed for carrying out a `cracking operation, the heat absorbed by the auxlliary c oil tempering 'the heating gases and reducing their temperature. In addition to the tempering action of the heat absorption in the auxiliary coil, hot gases at a lower temperature areadmixed with the heating gasesafter passing over the auxiliary heating coil whereby the heating gases are'furtherv cooled and tempered before passing over the heating surfaces of the stillpro er.

heating surfaces ofthe still no heat is lost in the tempering but an increase in both the capacity and efficiency of the st1ll may result. The heating'. surfaces of the still This dilution of' theheating gases wit hot gases of a lower proper JIAare thus protected in two ways, by

the tempering action of the heat absorption in, the4 auxiliary coil and by the tempering due to dilution of the heating gases -with hot gases, such as recycled waste heating gases, at a lower temperature.

The oil circulated through the auxiliary 'heatin coil is subjected to the lieatin gases at hig est temperature, that is itis subjected to the'most intense heat of the furnace. Substantially tar-free 0(1l1s employed for circulation Athrough the/auxiliary coil, a

such'for example as reflux, or' fresh oil,. or admixed reflux and fresh 0 11.; By using suchv substantially ytar free o1l :1n the auxiliary coil, the' danger of objectionable carbon deposit on the walls of. the. auxiliary heating coil is avoided Or minlmlzed, even though this coil is sub]ected to the `most 1ntense furnace heat, While at the Sametime the absorption of heat therein serves to temper the heating gases. v Moreover, according to the. present invention, the intense heat to which the oil circulating through the 4auxiliary coil is. subjectedis employed in 5 aparticularly advantageous manner for cracking 'reflux -and lighter constitutents of lfresh oil which if directly introduced, or reintroduced, into the pressure still proper mi httend to vaporize'before cracking abser ing heat in vaporization, which heat, in carrying out the present invention, is usefully applied in the cracking operation. The auxiliary heating coil is arranged to discharge into the pressure still proper, and, in conjunction with pressure stills provided with reflux` towers, the reflux, either with or without admixed freshoil, is' returned 'to the still proper through the auxiliary coil. The reflux towers may be operated at the same pressure maintained in the pressure still and the reflux, or admixed fresh oil and reflux, circulated through the auxiliary coil by means of gravity or convection, circulation or by means of alpump; `or the pressure on" the vapors vescaping from theA pressure still may be reduced and the vapors subjected to a reliuxing operation at a lower pressure and the reflux, or adlnixed fresh oil and'.v

-reliux, pumped through the auxiliary coil and back to the pressure still proper. The present invention is of special value and application'in connectionwith pressure stills provided with .reflux towers into which fresh oil is introduced and -from which the reflux and admixed fresh 'oil can be circulated through the auxiliary coil before Atheir introduction into the .pressure still proper.,

The present invention also is of special vvalue and `application, in connection with 40 vpressure still operations where the vapors from the pressure stillare subjected to ahigh degree of fractionation'and the invention provides. an advantageous method of handling reflux in such operations. The usual 'pressure distillates include large amounts of constituents heavier thansuitable` as comonents of the desiredfapressu distillate,

- or example, in cracking gasf oil to produce gasoline the pressure distillatey usually] in^- cludes a considerable amountof constituents of a kerosene character and others even heavier. If the vapors formin the pressure distillate are subjected to' aigh degree of fractionation, the reflux includes these 555 lntermediate constituents, and the method of the present invention isl articularly vadvantageous forhandling fe ux of this charac-v ter, the reflux being circulated through the auxiliarycoil where'it is subjected to the most intense heat and where the reflux, particu-Qa larly the :lighter constituentsjof t'hewrrelux.y aresubje'cted to cracking conditions without labsorbing heat in vaporizationV from the A 'maln charge off oil in the still proper. One .way of subjecting thevapors from the pres- `resulting improvement of the entire crack- ,special advantage for handling reflux. from t such operations. -ln such a combined operation, c lose control' ofthecliaract'er'of the pressure distillate is rcombined with an iinp'roved method of treating the lighter constituents of the escaping vapors which' are howeverv heavier than suitable as compo- L- nents ofthe desired pressure distillate with ing operation. i Accordingly, in carrying out`the `present invention, the heating surfaces of the pressure still proper are-protectedin a dual manner, the heating gases before` passing' over BJ the heating surfaces ofthe still prope1 being tempered rst by heat absorption in the auxiliary 'coil andsecond by dilution witlhotv gases ofa lower temperature, such as a recycled part. 'of the; waste heating gases, 0 while the intenseheat absorbed-in the auxiliary coil from the heating gases at highest temperature is employed in an advantageous `way in the cracking operation carried out in the pressure still system. 95 This invention will be furtherdescribed in "connection with the accompanying drawings illustrating apparatus embodying' the invention and adapted for. carrying out the process of the invention, but it will be understood that-the .invention is illustrated there- V by and not limited thereto. The accompanyi ing drawings show, in a diagrammatic and conventional manner, in elevation and partly in section vwith parts broken away, a tubular pressure still embodying the inventionandv adapted for the practiceof the process of the. y

invention. v

Whilethe invention is applicable. in connection with cracking stills of dilferent types yetit is of particular advantage in connection with tubular cracking stills of the character illustrated. This cracking still is .of the general-construction illustrated in United States Letters 'Patent No."1,285,200 granted to the SinclairRefining Company November 1 9, 1918 on the application of Edward W. Isom. The pressure still illustrated is made up ofthe bulk supply tank 1 located away from the heating furnace, the vertical heatcirculating the oil from the bulk supp y tank I upwardly through the vertical tubes and back to the bulk supply tank. 'L 1 Arranged above the bulksupply tanklis a reflux towerlhaving a series 'of bailes thereh k12in I ing'b tubes 2`arranged in the heating iue'of in and connected with the vapor dome ofv the bulk supply tank through the vapor line 8. An ovcrHow reflux line 9 leads from the re- Hux tower back to the bulk supply tank. The vapors from the top of the reHux tower escape through Qthe vapor line 10 to the condenser 11` which is arranged to discharge into the'recciver 12 from which uncondensed vapors and gases are drawn 0H through outlet 13 and liquid condensate through outlet 14; The pressure may be regulated andv reduced by means of a regulating valve 15 arranged between the reHux tower and the condenser, or by valves arrangedbeyond the condenser or receiver. A feed line 26 is provided for feeding. fresh `oil tothe top of the reHx tower 7 vAnother feed line 27 is provided for feeding cool oil to the bearings of feircu# lat-ing pump 6. `VAA tar draw-0H connection for use during the-operation of the still is `shown at 41 and a` tar draw-olf for -discharging the still is shown at 42.

LrIhe furnace constructionV for heating the tubes2 comprises a iire-boxl, suitably heat1 ed as by gas or oil burners 17, a vertical Hue 18 leading upwardly from the fire boxto the upper end'of the heating Hue in which the tubes 2 are arranged, and an outlet Hue 19 'leading to a stack from the lower end of the heating Hueqfor -causing the'heating gases Located' above the bridge wall ,19' and bet-Ween the vertical Hue 18 and the Hue containing the vertical tubes-2 is an auxiliary heating ,coil 20 hav.-

ing an inlet connecting pipe 21 and an outlet connecting pipe 22, the -l'atter being arranged i to discharge into the bulk supply tank 2() and the upper ends of the vertical tubes` through the circulating connection 5..` A

. blower 23 is varranged to withdraw wastet heating` gases .through an' outlet 24 located near the exit from the lower end of the heating Hue in `which the vert-ical tubes 2 are ar; i ranged and return them through inletj25 to a point in the- Hue between the auxiliary coil The blower illustrated is ofthe venturi ytype and maybe operated by a steam 4jet or 'other suitable means. Twov such blowers may be' arranged one on each side ofthe furnace structure'. o i

'An auxiliary tankor reservoir 28`is' arranged-to receive the reHux and admixed res oil from the reHux tower 7 -through connection 29. This tank or reservoir has a vent pipe .30 connecting with the reflux tower. The tank. or reservoir 28 is also connected throughithe pipes 31 and 32 tol the feed line 21 of the auxiliary` heating coil. A,

pump 33 isarranged to pump oil from the reservoir 28 through the auxiliary coil,while the pipe 32, -which'is provided with valve 34,

permits circulation by gravity and by convection when the pump l334 is not used.'` A

heatingflue. .BaHles are arranged in the' tion 38..;

to the inlet pipe 21 ofthe auxiliary coil and permits the direct introduction of fresh o il -at this point as it is desired. A pipe 39 having a valve 40 therein also connects the in-let of the auxiliary coil to the discharge side of the circulating pump 6. This pipe may bel used in starting the still in operation.

In the pressure still illustrated, the auxiliary reservoir or Jtank 28 is shown as located within thebulk supply tank 1, al-- though it is not in open communication there# with .except through thel pipe connections.` This location of the auxiliary lreservoir v1s sa a tank construction to begus'ed which does' fi" not need-to lwithstand high pressure as the not essential but is convenient and enables pressure inside and out-is approximatelythe same. lVhere this auxiliary tank or reservoir is located within' the bulk supply tank it will be evident that valves,V suchas valves 36 and 37, should have operating extensions extending through suitable stuHing'boxes. in the walls of the bulk supply tank. Test cocks (not shown) ma be pro'videdfor ascertaining the level o the oil in thls auxiliary tank or reservoir. i l

Whenthe still is initially charged 011 maybe circulated from the vmain charge'ofoil inl0 0 ing coil, for example, by closing valve 37- v the stillI proper through the auxiliary crackandopening valve 36when the circulating pump 33 will draw oil from the bulk supply.' f

tank and circulate it through v,theauxiliary coil, or-the valve 40 maybe opened and oil from the discharge side oftheclrculating pump 6 may be circulated through the aux- 4iliary coil, or fresh oil may be' circulated through the.' auxiliary coill from connecf- V y After the pressure-'still has been brought to crackingV conditions of temperature and i pressure, the introduction fof fresh Ioil into the top of the reflux tower through connect 4tion 26 is begun, The cracked vapors es lcaping.4 from the bulk supply tank rise through-the reflux tower land 'come-1n contact with this-fresh oil. Byvproviding a suflicie'nt number of bailles practically 'allof the feed for the still can be introduced, 1n this way.- The reHux and admixed -fresh oil,

from the bottom of the'reflux tower collect in v the reservoir or tank '28,which in a still of the type illustrated with a bulk supply tank having `a capacity for example 'of about 10,000 gallons, may have a capacity foi` ex- From this ample of about 1,000 gallons.

reservoir the oil is circulated. either-by gravityl or 'convection or by pump, through the auxiliary .heating coil 20 where it is sub4 i jected to the intense heat' of the heating gases substantially as thevleave the fire-box tank, or

' plied through connection 38.

' free of. tar constituents.

16. The auxiliary coil discharges into the bulk supply tank 1 through the top circulating-line. 5. The unvaporized portion of the oil so returned forms a .part of the still charge and is circulated therewith through .the vertical heating tubes 2. The vaporizcd part of this oil escapes w`iththe vapors from the bulk supply tank to the relux tower.y

It will be evident that the rate/of circulation of the'oil should be regulated so. that a constant supply is provided to the 'auxiliary coil. By introducing all or practically all of the fresh feed into the top of the reflux tower and bringing it into intimate contact with the vapors therein, afconstant supply of reflux and admixed fresh oil is provided. Should the reservoir 28become full the additional reflux and feed will collect in the bOttomof the relux tower and will overflow through the .connection 9 to the bulk supply part lof the reflux and admixed fresh oil collecting vin the reservoir may be discharged through ipe 35 into the circulatin line 3. Schoul there be any deficiency of oi inthe reservoir 28 to keep the auxiliary coil full, part of. the oilcirculating in the isti-ll properma be `supplied there- -through to pipe 39,*or fresh oil 'may be supv the auxiliary coil is thus made 'up of reflux constituents and fresh oil constituents and is substantially This oil also includesthe intermediate constituents refluxed. in the reflux towerwhich are light enough to be vaporiz'ed in the ressure still propenbut which are heavier t an desirable as components ,of the pressure distillate. These intermediate constituents, if returned directly to the charge'of oil in the bulk.` supply tank, tend to revaporize without further cracking,

The` oil^ circulating through absorbing .heat in a cyclic manner by vapor1. z ation in the bulk supply tank and condensation in the reflux tower which is not usefully applied in the cracking operation. By circulating the reflux through the auxiliary coil, these intermediate constituents are positively subjected to a cracking temperature the hottest heating gases substantially at the temperature at' which they leave the lire box,

and theserintermediate constituents, and any` lighter constituents of fresh oil circulated through the auxiliarycoihare'thus subjected to the mostv intense cracking conditions maintained in the still system.

The absorption of heat from the heating gases `inthe preliminary coil tempers .these gases, and, after. passing over the auxiliary coil, the; heating gases are further tempered by admixture therewith ofpart of the waste heating `gases returned from near the exit of heating iue of the furnace, and the heating gases are then circulated over theheating tubes through which the chargel of oil in the still proper is -circulated and recirculated. The charge of oil in the still proper, in whichl tar constituents tend'to accumulate, is .thus circulated through heating tubes over which heating gases tempered and reduced in temperature, both by the absorption of heat inthe auxiliary `coil and the recirculation. of;i flue gases, are passed, while the intense furnace heat, before tempering by recirculation of waste heating gases, is" employed'for heati'n and crackin cluding the intermediate constituents 'of the heating coil.

the reflux 9and fresh oil, in-.'

reflux and any lighter constituents of the fresh oil; circulatedthrough the .auxiliary Where the' oil circulated through the 'i auxiliary heating coil is fresh oil, or ad.- mixed freshoil and reflux, the auxillary C011 can be subjected to such high temperatures therein without objectionable carbon deposit on the walls of .the tubes of the coil; and

and a large amount of heat canbe absorbed after 'the high temperature furnace gases v have been reduced in temperature by passage' over the auxiliary coil, and further reduced in temperature by dilution with recirculated waste heating gases, they then pass over the heating tubes ,of the stilll proper at a lower temperatureand serve to heat these tubes without danger or with materially reduced danger .of objectionable overheating A milder heatingand cracking action is thus obtained in the still proper so that the still charge can be cracked to a further extent without danger. of objectionable carbon/deposit on the tubes lwhile obtaining a` heavier residuum from the cracking operation. At the same time, the higher temperature to which rthe auxiliary cracking coil is subjected also permits the cracking therein'of lighter oils, lsuch as kerosene frac tions, which are difficult 'to crack in ordinary pressure stills, and enables the advantageous further treatment of intermediate constltuents produced by thepressure still proper and refluxedin the reflux tower within the still system. 5 c

{Ihe heating surfaces of the still proper may be further protected by withdrawing tar laden oil from the main' still charge during the progress of the cracking operation,- for example through connection 41, and` stipplying suiicient additional fresh oil to maintain the still charge. The rate of f tion during all or the greater part of the run, or the tarmay be allowed to accumulate in the recirculating still charge, either duringthe-run or toward the end of the run,

.Ia-iid the residuuin in the lstllproper reduced to a lower gravity without detriment to the operation. In cracking gas oil character charging stocks vfor tliefproduction of gasoline, for example, after initiallyA charging the still `with gas oil, the gas oil feed introduced stituents circulated ,through the auxiliary' coil. For a higher degree of fractionation of the vapors escaping from the pressure still, a part of the-pressure distillate, or a fraction of corresponding character, may be l supplied to the upper end ofthe reiiux tower through connection 26 in admixture with the gas oil. In this case, the reflux includes, in

addition to .the heavier intermediate constituents, lighter intermediate constituents coil, and gas oil charging stock4 also being v supplied to the auxiliary coil through connection 38 or introduced directly into the charge of oil in the still proper, for example through the bearings of the circulating pump by means of connection 27. Kerosene A 'ternallyheated pressure stills for crac ingcharacter fractions may also be cracked in the auxiliary coil in such an operation. For

example, if the normal pressuredistillate includes lkerosene as wellas gasoline constituents, the pressure distillate bottoms produced on fractionation of the .pressure`distillate for *the desired 'gasoline fraction may be supplied to the 'auxiliary coil through ,connection 38, or kerpsene character stocks from other operationsy may be subjectedto cracking treatment inthe auxiliary coil. lWrhere v the yreflux tower vis doperated with ia high degree of fractionation, kerosene character l A constituents produced in the `still are returned to the auxiliary coil with theirefiiix.

-The temperature and pressure 'employed in carrying out the invention can be varied with different characters of charging stock. Where ordinary gas oil is employed as the principal charge in the pressure proper, pressures of around 90 to .1254 lbs. per sq. n.

maintained at a` considerably higher pressure and the pressure reduced as the cracked oil is discharged therefrom into the pressure still proper. Iclaim:` i' 1. An improved method of operating ex-v ternally heated pressure stills for cracking hydrocarbonoils, which comprises passing heating gases for: supplying heat to the cracking operation first overl an auxiliary heating coil and then over the heating surfaces ofthe pressure still proper, 'admixing hot gases ata lower temperature than the said heating gases with 'the said heating gases at a point in their passage between the auxiliary heating coil and the heating sur faces of the still roper, and circulating oil through said'auxiliary ,heating coil' to the 'pressure still proper.

2. An improved method 'of operatin externally heated pressure stills for "crac ing' .hydrocarbon oils, Which comprises passing heating gases for supplying heat to `the cracking 'operation first over an auxiliary heating 4coil and tlienover the heating surfaces of the .pressure still proper, recycling a part'of the heating gases which have passed over the heating'surfaces of the pressure still proper and admixing them withl the f heating gases passing from the auxiliary heating coil to the heating surfaces lof the pressure -still proper, and circulating oil through said auxiliary heating coil. tothe .pressure still proper.

3. xAn `improved method of operatin Y exhydrocarbon oils, whichl comprises passing heating gases for supplying heat to the cracking operation first over anv auxiliary heating coil and then over the heating surfaces 'of the pressure still proper, admixing hot gases at a lower temperature than the said heatpoint in their passage between t e auxiliary -heating coil and the heating surfaces ofthe still proper,- and circulating substantially tar free oil through the said auxiliary heat-- ing coil tothe pressure still proper.v

4. 'An improve method of operating exterf 'nally heated pressure stills for crackingl hyl drocarbon oils, which comprisespassing veat- .ing gases for supplying heat to the crackin, operation first over an auxiliaryvheati'ng coi l and then over the heating surfaces of the pressure still proper, admixing hot gases at a lower ztemperatl'ire than the said heating gases with m35 be employed, while if lighter roils form. the said heating gases attua point in their the principal part of the charging stock of passage 4between the auxiliary heating coil gressure still proper, higher pressures and the heating surfaces of the still proper, e used, for example, ,of around 300 to subjecting the vapors from the pressure still 325 lbs. per sq. in. The pressure maintained proper to a reiuxing operation, and clrculating gases with the said heatingi gases at alll in the auxiliary cracking coil may be. the ing reflux therefrom through the said -aux'- 4 same as that in the pressure still or only iliary heating coil to the pressure still proper.`

enoughhigher to cause the desired circula- 65 tion; or the auxiliary cracking coil may be ternally heated pressure stills for crac iig l 5. Animp'roved method of operatn exhydrocarbon oils, which comprises passing heating gases for supplying heat to the cracking operation first over an auxiliary heating` coil and then over the heating sur- 'faces of the pressure still proper, recycling a part of the heatingl gases which have passed over the lheating surfaces of the pressure still proper and adinixing them with vthe heating gases passing from the auxiliary heating coil to the heating surfaces ofthe still proper, subjecting the vapors from the still properto a reiluxing operation, and circulating reflux therefrom through the said auxiliary heating coil to the pressure still proper."v

6. An improved method of operating cx? ternally heated pressure stills for cracking hydrocarbon' oils, which comprises passing the heating gases for supplying heat to, the cracking operation first over an auxiliary heating coil and then over the heating surffaces of the pressure still proper, admixing hot gases at a lower temperature. than the said heating gases with the said heating gases-at a point in their passage between the-A auxiliary heating coil andthe heating sur` faces of the still proper, subjecting the vapors from'thepressure` still proper to a re-v iuxing operation in direct contactwith-fresh oil, and circulating the reflux and fresh oil-n from the reiuxing` loperation through the said auxiliary heating coil 'to the pressure Astill proper.

over the heating surfaces of the pressurestilL proper and admixing them with the heating gases passing from the auxiliary heating coil to the heating surfaces of the still proper, and circulating substantiall tar free oil through saidauxili'ary heating coilV to the( pressure still proper.

f 8. An improved method of operating ex ternally 'heated pressurestills f rv cracking hydrocarbon oils, which comprises passing heating gases for supplying heat to the cracking operation first over an auxiliary` heating coil and thenl over the heating surfaces of the pressure still proper, admixing h`ot gases at a lower temperature than the said heating gases with the said heating gases at a point in their passage between the auxiliary heating coil and the heating'surfaces of the still propers subjecting the vapors from the pressure still proper toi a refluxing operation in direct contact witha fraction corresponding in character to the desired pressure distillate, and circulating reflux .from the refluxing operation through the said auxiliary heating coil to the pressure still proper;

9. An improved method of operating er@ ternally heated pressurev stills for cracking hydrocarbon oils, whichcomprises passing heating gasesforv supplying heat to the crackingv operation-first over an auxiliary heatingcoil and then over the heating surfaces of the .pressure still proper, admixing hot gases at a lower 'temperature than the vsaid heating gases with the said heating gases at a point in their passage between the auxiliary heating coil and the heating lsurfaces of the still proper, subjecting the vapors from the pressurestill proper to a reiuxing operation in direct contact with a fraction corresponding in character to the desired pressure distillate, and circulating the reflux therefrom together with additional fresh oil through thesaidauxiliaryheatingcoil to the pressure still proper.

10. An improved method of operatlng externally heated pressure stills `for cracking hydrocarbon oils, which comprises` passing heating gases. for supplying heat to the cracking operation rstover an auxiliary heating coil and ,then over the heating sur- 90 faces of the pressure still proper,.admixing hot ases at a lower temperature than the said. eating gases with the said heating gases at a point in their passage intermediate the auxiliary heating coil and the heatingo surfaces of the still proper, and circulating substantially tar free oil through the said i auxiliary heating coil. 1 f

11.. Animproved method of operatin exf' ternallyI heated pressure stills for crac g 100 hydrocarbon oils, which comprises passing heating gases -for supplying heat tothe v'cracking operation first over an auxiliary heating coil and then over the heating surfaces of the ressure still proper, recycling a part of t e heating" gases which have passed ver`the heating surfaces of the pressure still proper and admixing them with the heating gases passing from the auxiliary heating coil to theheating surfaces of the 11o still proper, and circulating substantially tar free oilthroughthe said auxiliary heatr ing coil. u

12. In an externally vheated voil-'cracking pressure still, a heating furnace comprising 11 5. a heating flue with the heating surfaces of the still arranged therein and having a firebox connected thereto, an auxiliaryheating coil arranged to discharge into thestill and interposed in the fluebetween the lire box and the heating surfaces of the still, and means-for introducing hot tempering gases into the heating fiue between the auxiliary healting coil and the heating surfaces of the sti v 13. In an externally heated oil cracking pressure still, a heating furnace comprising I a heating Hue with the heatin 'surfaces of the still arranged therein and havin a firebox connected thereto, an auxiliary eating 130 coil arranged to discharge into the still and interposed in the iiue between the firebox and the heating surfaces of the still, and means for recycling heating gases from the flue be-V xyond the heating surfaces of the still and returning themto the flue' between the auxiliary heating coil and the heating surfaces of the still.

14. In externall heated oil cracking pressure stills comprising a bulk supply tank and heating tubes with connections for circulating oil from the bulk supply tank through the heating tubes and back to the bulk supply tank, a heating flue with the heating tubes of the still arranged therein having a lirebox connected thereto, an auxiliary heating coil arranged to discharge into the still and interposed in the flue between the firebox and the heating tubes of the still, and means for introducing hot tempering gases into the heating flue between the auxiliary heating coil and the heating tubes of the still. y

15. In externally heated oil cracking pressure stills, a reflux tower connected to the still, a heating iue with the heating surfaces of the still arranged therein and having a firebox connected thereto, an auxiliary heats ing coil arranged to discharge into the still and interposed in the flu between the lirebox and the heating surfaces ofthe still, means for introducing hot tempering gases intothe heating flue between the auxiliary heatin coil and the heating surfaces of Athe stil, and "Connections for circulating reflux from the reflux tower through the auxiliary heating coil.

16. In externally heated oil cracking pressure stills, a reiiux tower connected to the still, means for introducing fresh oil directly into the reflux tower, a heating flue with the heating surfaces of the` still arranged therein and having a firebox connected thereto, an auxiliary heating coil arranged to discharge into the still and interposed in the lflue between the frebox and the heating surfaces of the still, means for introducing hot tempering gases into the heating i'lue between the auxiliary heating coil and the heating surfaces of the still, and means for circulating reflux and admixed fresh oil from the retllux tower through the auxiliary heating co1 In testimony whereof I affix my signature.

EUGENE C. HERTI-IEL. 

